1. Field of the Invention
This invention relates generally to power switches, and relates more particularly to a circuit for automatically switching between line-driven and battery power supplies.
2. Description of the Relevant Art
A combination of battery and line-driven power supplies are used to power many electronic circuits. In some electronic circuits, it is necessary to provide a continuous supply of power to permit continuous operation of the circuit. For example, a real-time clock circuit requires a continuous supply of power to ensure accurate time keeping. In other electronic circuits, it is desirable to operate from line power when available, and to output from a battery when line power is not available. Portable electronic products are examples of the latter type of circuit. In either case, a battery-driven power supply is utilized in conjunction with a line-driven power supply to provide power to the circuitry. Two approaches have been used in prior art circuits: independent power supplies, and externally switched power supplies.
In a circuit using independent power supplies, a battery provides power to a portion of the circuit that requires continuous power, while a line-driven power supply provides power to the remainder of the circuit. The battery powered portion of the circuit is independent from the line-driven portion of the circuit. As a result, the battery is continuously in use, regardless of whether the line-driven power supply is on or off. Level shifting circuitry is required if the battery voltage is less than that of the line-driven power supply, as is commonly the case, in order to exchange data between the two portions via an internal data bus. Since the battery powers the internal data bus circuitry, exchange of data between the two portions of the circuit involves additional power drain on the battery.
There are several disadvantages to using independent power supplies. One disadvantage is that, since the battery continuously powers a portion of the circuit, there is a large power drain on the battery. Another disadvantage is that, since the battery powers the internal data bus circuitry, access times for the battery powered portion of the circuit are constrained by the need to conserve battery power. A further disadvantage is that independent power supplies are not feasible for portable products due to the large and continuous power drain on the battery. Still another disadvantage is that data may be lost when batteries are exchanged. The use of independent power supplies does however have certain advantages. Separate circuitry for switching the battery power supply on and off is not required, and the full output voltage of the battery is available for use.
In a circuit using externally switched power supplies, a battery power supply and a line-driven power supply are routed through a switch. The switch isolates the battery and connects the line-driven power supply to the circuit when the line-driven power supply is on. When the line-driven power supply is off, the switch connects the battery to the circuit and isolates the line-driven power supply. Such an approach conserves battery power because the battery is not used when the line-driven power supply is available. Access times can be fast because line power is used to power the access circuitry. This approach does, however, have disadvantages. If the circuit is implemented on an integrated circuit chip, the switch is typically located external to the chip, with the result that circuitry of the switch is costly to implement. Another disadvantage is that the battery is usually isolated by a diode, which dissipates a significant amount of battery energy during battery powered operation.